Many valve assemblies for regulating fluid flow retain a certain volume of fluid within the valve, which is typically referred to as dead volume. Dead volume can be a particular problem when the valve is installed in apparatus for metering relatively small, exact quantities of fluid, such as hematology and immunology testing apparatus, and other apparatus for fluid-particle analysis. As one example, a typical miniature solenoid valve assembly used for regulating the flow of reagents or blood samples in hematology testing apparatus may have a dead volume on the order of approximately 20 .mu.l. Accordingly, with relatively small reagent or blood volumes, on the order of 60 .mu.l, for example, the dead volume of the valve can consume and waste a substantial portion of the sample. Moreover, if several such valve assemblies are used within the same apparatus, the total dead volume of the apparatus increases with each additional valve, which can in turn lead to a substantial waste of the fluids being tested.
In FIG. 1, a typical prior art solenoid valve assembly is illustrated schematically and indicated generally by the reference numeral 1. The valve assembly 1 includes a port head 2 mounted on a solenoid 3. The port head defines an inlet port 4, an outlet port 5, and a valve seat 6 formed on the inner side of the port head between the inlet and outlet ports. A flexible diaphragm 7 is seated between the port head 2 and solenoid 3, and is sealed along the diaphragm's peripheral edge. A valve rod 8 is coupled between the diaphragm 7 and solenoid 3 for moving the diaphragm relative to the valve seat to open and close the valve, as indicated by the arrows in FIG. 1. A spring 9 is coupled to the valve rod 8 and normally biases the rod and diaphragm into engagement with the valve seat 6 to close the valve, as shown in FIG. 1.
The valve assembly is opened by energizing the solenoid 3, which drives the valve rod 8 downwardly in FIG. 1, and in turn moves the diaphragm 7 away from the valve seat 6, thus opening the passageway between the inlet port 4 and outlet port 5 to permit fluid flow through the valve assembly. The valve assembly may then be closed by de-energizing the solenoid, which in turn permits the spring 9 to drive the valve rod 8 and diaphragm 7 into engagement with the valve seat 6, as shown in FIG. 1, thus closing the passageway between the inlet and outlet ports and preventing fluid flow through the valve assembly.
In the example of FIG. 1, the valve assembly 1 is arranged to transfer fluid with a syringe or like pump A from a first vessel B into a second vessel C, and thus the outlet port 5 of the valve assembly is coupled between the syringe and second vessel by a T-fitting D. In order to draw the fluid from the first vessel B, the solenoid 3 is actuated to drive the diaphragm 7 away from the valve seat 6, thus permitting the syringe pump A to draw the fluid through the inlet and outlet ports, and in turn through the T-fitting D and into the tube leading to the pump. Then, the valve assembly is closed by driving the diaphragm 7 into engagement with the valve seat 6, and the syringe A is actuated to pump the fluid back through the T-fitting D and into the second vessel C.
As can be readily appreciated, after the fluid is drawn from the vessel B into the tube leading to the syringe pump A and the valve assembly is closed, a volume of fluid is trapped in the dead space between the diaphragm 7 and T-fitting D, thus defining the dead volume of the valve assembly, as indicated in FIG. 1. Even if the T-fitting D were moved as close as possible to the outlet port 5, a dead volume between the outlet port and diaphragm would still exist. Not only can this dead volume be relatively substantial, particularly when the volume of a fluid sample is relatively small, but the dead volume may vary from one valve assembly to another, and from one instrument to another, thus leading to inconsistencies and uncertainties in test results.
Accordingly, it is an object of the present invention to provide a valve assembly having a substantially zero dead volume, and which thus overcomes the drawbacks and disadvantages of such prior art valve assemblies in a simple and cost-effective manner.